The present invention relates to the field of electronic devices and materials. More specifically, the present invention relates to dielectric composite materials having a polymeric matrix comprising dense ceramic beads dispersed therein and further relating to the methods for making the ceramic beads.
In the field of electronic devices, there is a need for materials having a relatively high dielectric constant for passive circuit elements such as capacitors, filters, and the like. It is particularly desirable to have such materials in forms that comprise a polymer matrix (as a thin, flexible sheet or as a printable composition) in order to incorporate capacitive elements into polymer thick-film circuits and other low-cost electronic assembly methods. The simple incorporation of dielectric powder, such as barium titanate, into a polymer binder is limited to some degree by the volume fraction of powder that can be incorporated while maintaining adequate; fluidity or workability of the material. This can be alleviated to some degree by proper control of particle size distribution (J. J. Felten, U.S. Pat. No. 5,744,285). It would be useful to have a material that has a high dielectric constant wherein the material comprises dense, ceramic microspheres that could easily be controlled and manipulated to solve many of these problems.
Accordingly, it is an object of the present invention to produce spherical beads of ceramic dielectrics for passive electronic devices.
It is another object of the present invention to produce large numbers of substantially mono-sized dielectric beads for incorporation into a polymer matrix or coating.
It is yet another object of the present invention to produce dielectric beads containing selected dopants to modify the dielectric properties.
It is a further object of the present invention to produce dielectric beads by hydrothermal reaction of titanium oxide (TiO2) or zirconium oxide (ZrO2) with selected soluble salts, particularly alkaline earths.
It is still yet a further object of the present invention to produce polymeric films having ceramic dielectric beads dispersed therein.
It is another object of the present invention to produce polymeric films having dispersed ceramic dielectric beads that are capable of being electrically poled.
It is a further object of the present invention to produce capacitors and filters in which the dielectric medium is a polymer composite comprising dense dielectric beads.
Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a dielectric medium comprising a polymeric matrix having ceramic spherical beads dispersed therein, wherein the spherical beads have a substantially high density and wherein the polymeric matrix has a dielectric constant  less than 20 and the spherical beads have a dielectric constant  greater than 100.
In accordance with another aspect of the present invention, other objects are achieved by an electrical capacitor comprising a dielectric medium disposed between two conductive electrodes wherein the dielectric medium comprises a polymeric matrix having sintered ceramic spherical beads dispersed therein. The spherical beads have a substantially high density and the polymeric matrix has a dielectric constant  less than 20 and the spherical beads have a dielectric constant  greater than 100.
In accordance with yet another aspect of the present invention, other objects are achieved by a prefired ceramic dielectric for polymer thick-film capacitors comprising a polymeric matrix having sintered ceramic spherical beads dispersed therein. The spherical beads have a substantially high density and the polymeric matrix has a dielectric constant  less than 20 and the spherical beads have a dielectric constant  greater than 100.
In accordance with still yet another aspect of the present invention, other objects are achieved by a method for making a dielectric composite material comprising the steps of forming spherical hydrous metal oxide beads by a sol gel process wherein the beads comprise at least one Group IVb metal species. Then calcining the beads to form sinterable, spherical beads and sintering the spherical beads to a desired density and a desired grain size. Then dispersing the beads in a polymer matrix to form a dielectric composite material.
In accordance with another aspect of the present invention, other objects are achieved by a method for making an electrical capacitor comprising the steps of forming spherical hydrous metal oxide beads by a sol gel process wherein the beads comprise at least on Group IVb metal species. Then calcining the beads to form sinterable spherical beads and sintering the spherical beads to a desired density and a desired grain size. Then dispersing the beads in a polymer matrix to form a planar dielectric composite material and applying electrodes to opposite surfaces of the dielectric composite material.